MINERAL PROCESSING: Optimization starts with blasting and crushing

GHANA - Metso Process Technology and Innovation (PTI) recently conducted a mine-to-mill process integration and optimization (PIO) project at the Newmont Ahafo mine in Ghana. The result of this project was a significant increase in mill throughput without capital investment. This was achieved by reviewing the current operating conditions, identifying inefficiencies and then implementing PTI's recommended changes in drill & blast, crushing and grinding practices.

Ahafo mines number of open pits, with a blend of both primary and oxide ore fed to the mill to balance the head grade and recovery in the leach circuit. Ahafo operate a 1,370x1,905-mm gyratory crusher for primary ore and a toothed roll, sizer for oxide ore. This is followed by an SABC circuit comprising a 10.4x5.0-m, 13-MW semi-autogenous grinding (SAG) mill in closed circuit with 2,60-kW pebble crushers. SAG milling is followed by a 7.3x11.9-m, 13-MW ball mill in closed circuit with a cluster of 660-mm cyclones. After classification, the milling product is thickened and the overflow is fed to carbon-in-leach tanks.

A Metso PIO project is normally comprised of a number of site visits spaced over a few months. The first site visit is to establish current operating practices, initiate rock characterization and collect measurements of blast fragmentation and mill performance during a series of plant trials or sampling surveys. This is followed by modelling and simulation studies to determine how to best exploit hidden inefficiencies. These recommendations are then followed by further site visits to implement the changes, monitor the results and ensure the improvements are maintained over time.

In a PIO project, it is important that the material being surveyed in the plant is the same that has been characterized and closely studied from the audited blast. This requires ore tracking. PTI have developed a radio-frequency based material tracking system called SmartTag^TM, that allows parcels of ore to be followed from the mine through the crusher, intermediate stockpiles and finally into the grinding circuit. The RFID tags are detected as they pass near antennae that are normally installed on conveyor belts. Being passive, the tags do not have a power source and therefore can last an indefinite period and reside in stockpiles for many years if necessary. Detecting tags before and after stockpiles can provide a reliable indication of their active residence time.

Two full grinding circuit surveys were performed at Ahafo as part of the initial benchmarking phase of the PIO project. Both surveys were completed while processing material from a particular blast, tracked using SmartTags^TM. At Ahafo, each blast is mined in two benches, and the first survey was conducted on the top bench while the second was done on the bottom bench. This would clearly show the effect of the coarser, top bench fragmentation on mill throughput.

At the end of each survey, the SAG mill was crash-stopped and the rock and ball loads were measured. The survey sample sizing data were then used for mass balancing and calibration of crushing and grinding circuit steady-state models.

Blast design changes were modelled and the predicted run-of-mine fragmentation 'fed' to the grinding circuit model during a number of simulations to achieve different increases in mill throughput. The blast patterns were designed to increase the amount of ultra fines generated during blasting that cannot be created through conventional crushing. These very fine particles are beneficial to both semi-autogenous and fully autogenous milling as they pass through the mill and onto the ball mill circuit. Once the primary mill restriction has been alleviated, issues around the ball mill will need to be addressed as this can become the bottleneck. For the Ahafo project, the blast designs recommended by PTI aimed to increase the amount of ultra fines from 18% to 24%.

As well as blast design changes, the performance of the crushing and grinding circuits were reviewed during the PIO project. It was recommended that Ahafo measure the primary crusher closed-side-setting once a week and maintain it between 125 and 152 mm, based on operating power draw. This would ensure the additional fines generated in blasting would be supplemented with good top size control by the primary crusher.

Other PTI recommendations included operation of the pebble crusher, SAG mill rock and ball load monitoring, ball mill media size and cyclone conditions.
Following implementation of PTI's recommendations, plant trials conducted in the last quarter of 2010 showed a significant increase in mill throughput compared to the benchmark surveys on the same material earlier in the year. Metallurgical hardness tests showed that the validation trial mill feed was even harder than the benchmarked ore.

Completion of Metso PTI's mine-to-mill PIO project at Ahafo has resulted in sustained improvements in drill and blasting practices and process plant throughput.

The combination of Metso's PIO methodology along with active engagement and cooperation of Newmont Ahafo personnel ensured the success of this project in under a year. Ahafo are currently implementing further PTI recommendations to develop standard operating practices for both SAG and ball mill circuits.

___________________________________________________________

The preceding article was supplied by Metso with the permission and co-operation of Newmont Ghana Gold. Please see www.Metso.com.